The invention relates to a system for recording and/or detecting marking signals of two different types which can be recorded on or scanned from a record carrier and which differ from each other in respect of at least one parameter, which system comprises a recording device for recording the marking signals and a detection device for detecting said signals, the recording device comprising generator means for selectively generating the marking signals to be recorded on the record carrier and the detection device comprising detector means for selectively detecting the marking signals read from the record carrier.
In a known system of this type, as for example used in a dictation machine for recording and playing back dictations which is manufactured and marketed by the N. V. Philips Gloeilampenfabrieken of the Netherlands under the type designation LFH 0303, the two marking signals are sinewave signals or different frequencies. One marking signal is a sinewave signal having a frequency of 40 Hz and the other marking signal is formed by a mixture of two sinewave signals having frequencies of 40 Hz and 1500 Hz. The marking signals identify specific locations where information signals corresponding to the dictated text are stored on the record carrier, such as for identifying the ends of dictations or information-signal locations corresponding to passages of the dictations to be corrected.
In the known system the generator means comprises two sinewave generators for generating the marking signals and the detector means comprise two amplitude detectors for detecting the marking signals. To ensure correct and reliable operation, filter circuits are required both for the generation and the detection of the sinusoidal marking signals. These filters are comparatively intricate and expensive, occupy a substantial amount of space and in addition require frequency adjustment. Moreover, in such a system in which the frequencies of the marking signals differ from each other, the spacing between the marking signal frequencies must be comparatively wide for reliable and correct detection of signals. As a result of this at least the frequency of one of the marking signals will be situated within the frequency range of the information signals which can be stored on the record carrier. Thus the information signals may be disturbed by the relevant marking signal and the information signals may adversely affect the detection of the relevant marking signal. Moreover, when marking signals of different frequencies are detected, the frequency of the marking signals scanned from the record carrier depends on the speed of transport of the record carrier relative to the scanning element by which the marking signals are scanned. A variation of this speed of transport during scanning of the marking signals or the use of a speed of transport during scanning of the marking signals which differs from that during recording, for example when the marking signals are scanned in a fast-wind mode of the record carrier, then causes the frequencies of the marking signals scanned from the record carrier to be shifted relative to the frequency selected during recording. Allowance for this shift must be made in processing the marking signals further complicates the circuitry.